1. Field of the Invention
The present invention relates to an output signal level control circuit, and more particularly, to an output signal level control circuit for controlling an output level of an output signal with a wide dynamic range when the output signal level control circuit is applied to a cellular telephone system and so on.
2. Description of the Related Art
In a car telephone apparatus, the transmission output level of an output signal must be maintained. For this reason, the output level of the output signal is always monitored and controlled with high precision in a wide range. In general, when the output level of the output signal is controlled, a part of the output signal is branched and detected by a detector. A variable attenuation unit or an amplifier is controlled based on this detection result.
In this case, it is difficult to control the output level of the output signal with high precision in a wide range because of non-linearness and temperature change of the detection characteristic of the detector.
A conventional example of the output signal level control circuit which solves such a problem is disclosed in Japanese Laid Open Patent Disclosure (J-PA-Heisei 7-106881). According to this reference, the first and second level control circuits are provided to weight the detection outputs of detecting circuits on the low level side and the high level side, respectively. By appropriately switching between the first and second level control circuits by a switching circuit, the output level of the output signal can be controlled with stability and high precision.
However, in the above-mentioned conventional example of the output level control circuit, the first and second level control circuit need to be adaptive for the detection characteristics of the detecting circuit. For this reason, the adjustment of the inclination of the detection characteristic curve and the direct current offset quantity becomes complicated and took a long time.
Also, because it is necessary to provide the memory which previously stores the detection characteristic of the detecting circuit so that the circuit scale of the detecting circuit becomes large, there is a problem in low production cost and manufacture with a small size.
Further, in the output signal level control circuit which performs the weighting of the output signal on the low level side, because the level control is performed with a high gain to correct the low detection efficiency of the detecting circuit in the low level, it is necessary to perform temperature compensation to prevent an error due to temperature change.